Phase modulator for fibre-optic gyroscope, fibre-optic gyroscope and method for operating a phase modulator

ABSTRACT

The control system for a fiber-optic gyroscope comprises a phase modulator for modulating a phase of a light signal and a control unit for producing a control signal, by the magnitude of which the phase is modulated and which is fed to the phase modulator. Moreover, the control system comprises an integration unit for determining an integral value of an integral over an input signal. Here, the control signal assumes a first value or a second value depending on the integral value.

RELATED APPLICATIONS

The present invention is a U.S. National Stage under 35 USC 371 patent application, claiming priority to Serial No. PCT/EP2017/058677, filed on 11 Apr. 2017; which claims priority of DE 10 2016 107 561.2, filed on 22 Apr. 2016, the entirety of both of which are incorporated herein by reference.

The invention concerns a control system for a fiber-optic gyroscope as well as a fiber-optic gyroscope, in particular a fiber-optic Sagnac interferometer and a method for operating a phase modulator of a fiber-optic gyroscope.

The invention concerns moreover a control system for a fiber-optic current sensor based on the Faraday effect as well as an according fiber-optic current sensor and an according method.

Fiber-optic gyroscopes such as e.g. Sagnac interferometers are used in rotation rate sensors of inertial navigation systems.

Inertial navigation systems are realizable in various manners, wherein typically capturing of forces or accelerations acting on an object and the resulting rotation rates are the basis for determining a position. Instead of mechanical effects also optical effects can be used for determining the rotation rates for inertial navigation systems. Such an inertial navigation system may be based on at least one fiber-optic gyroscope such as a Sagnac interferometer. Here, the Sagnac effect is used, according to which, during a rotation of an optical light guide loop around its normal, an optical path difference between two light beams running oppositely through the light guide loop occurs. In observing the light of the two light beams that exits the light guide loop in superposed manner during the rotation an intensity relation is visible that can be described by an interferometer characteristics that indicates the variation of intensity depending on the phase difference between the two light rays. Stated differently, a rotational movement acting on a fiber-optic gyroscope such as a Sagnac interferometer causes a phase shift between the oppositely running light beams such that at the location where the two beams are superposed a variation in intensity that depends on the rotational movement can be observed.

The phase shift in a fiber-optic Sagnac interferometer is directly proportional to the rotational velocity, the length of the light path in the light guide loop or a light guide coil, and the diameter of the circular light path. The phase shift is in addition inversely proportional to the wavelength of the used light.

The interferometer characteristics as mentioned above that indicates the dependency of the light intensity from the phase difference, which light intensity shall serve as observation variable for determining the rotation, has a cosine shape.

As a corresponding transfer function is insensitive with respect to small input values at the maximum of the cosine curve, and as a sign of the phase shift corresponding to the direction of rotation cannot be determined, typically a working point of the fiber-optic gyroscope is adjusted by phase modulation such that it is located at a point of maximum gradient of the cosine function. To this end, for example sine wave or square wave modulation may be used. This is done to ensure already at small rotational movements a maximum sensitivity of the interferometer.

A rotation rate sensor that contains a fiber-optic gyroscope comprises often a multifunctional integrated optical chip (MIOC) as phase modulator by means of which a phase modulation of passing light beams can be achieved. The MIOC is typically part of a control loop for adjusting the aforementioned phase modulation by means of a control signal. Physical effects within the MIOC, e.g. movable charge carriers, cause a dependency of the phase modulation on the frequency of the control signal. The MIOC has therefore a frequency response due to which for different frequencies different frequency responses occur. In particular, the response behavior of the MIOC for small frequencies is less strong than for large frequencies. As this phenomenon is also based on the movability of charge carriers within the MIOC, a dependency of the MIOC frequency response on the surrounding temperature or the temperature of the MIOC itself is present.

This MIOC frequency response leads to a specific form of the lock-in effect that causes accumulation of output signals that correspond to a rotation rate of 0°/h, while actually small rotation rates around 0°/h are present. The resulting insensitivity of a fiber-optic gyroscope such as a Sagnac interferometer with respect to very small rotation rates obstructs a highly precise and reliable measurement, in particular of very small rotation rates.

The invention is concerned with the problem to provide a control system for a fiber-optic gyroscope for which the lock-in effect caused by the frequency response of the phase modulator is effectively limited and that can be used to carry out highly precise rotation rate measurements even for small rotation rates. According to the present invention this problem is solved by the subject-matter of the independent claims. Further embodiments are indicated in the respective dependent claims.

A control system for a fiber-optic gyroscope comprises a phase modulator for modulating a phase of a light signal and a control unit for generating a control signal by the value of which the phase is modulated and that is fed to the phase modulator. Further, the control system comprises an integrating unit for determining an integral value of an integral over an input signal. Here, the control signal takes on, depending on the integral value, a first value or a second value.

It was possible to show that by using a control signal for phase modulation in a fiber-optic gyroscope, whose values are determined based on an integral value of an integral over an input signal, the low frequency part in a control signal can be reduced. Control signals for small rotation rates have therefore a similar frequency spectrum as control signals for large rotation rates. By using such control signals, also for small rotation rates it is ensured that no control signals with low frequency components are fed to the phase modulator for controlling the phase modulation. Hence, the response behavior of the phase modulator for small rotation rates approximates the response behavior of the phase modulator for large rotation rates, due to which the lock-in effect is reduced and a preference of the measurement value “0°/h” for small rotation rates vanishes.

The control signal may take on the first value and the second value such that it is zero-mean. By using a control signal of mean zero, the lock-in effect will be suppressed.

The control unit may generate the first control signal from the input signal. This ensures that the control signal is deduced from a known input signal. The first value of the control signal may for example correspond to the value of the input signal, while the second value of the control signal corresponds to a value that is shifted by the fixed amount from the value of the input signal. This means that the phase modulation caused by the first value is the same as the phase modulation caused by the input signal, while the phase modulation caused by the second value of the control signal is additionally shifted by a fixed amount. The statistical and zero-mean control signal is therefore determined from a predetermined input signal.

The phase modulation signal may for example be used to determine at which phase or phase difference the intensity of the interference signal determined in the fiber-optic gyroscope shall be measured, i.e. the phase modulation determines the working points of the intensity measurement at the interferometer characteristics. The input signal may then e.g. be used to determine a working point. The input signal is then transformed such into the control signal that the working point determined by the input signal or other working points are selected that give equivalent measurement results. This ensures that despite the control signal used for reducing the lock-in effect measurement parameters for the intensity measurement can be defined.

Here, the control unit may generate the control signal with the first value from the input signal, when the integral value is smaller than or equal to a switching threshold, and may generate the control signal with the second value from the input signal, when the integral value is larger than the switching threshold. This allows setting up clear conditions under which the first or the second value will be used. When the integral over the input signal becomes too large, the control unit switches from the first value to the second value. By selection of the switching threshold for which this switching is carried out, it can be ensured that the low frequency part of the phase modulation signal is eliminated, which reduces the lock-in effect.

Here, the integrating unit may be a delta-sigma-modulator. Due to this, the temporal mean of an output bit stream becomes equal to the mean of the input data. This guarantees that the control signal is zero-mean.

The control unit may reduce the integral value also by a predetermined amount. This ensures that the control unit can influence whether the phase modulator receives the first or the second value of the control signal, since the control unit can reduce the integral value. The control unit may for example reduce the integral value by a fixed or temporally varying amount, when a predetermined condition occurs, which may also result in a change of the control signal used for phase modulation.

The control unit may for example reduce the integral value by a predetermined amount, when the integral value exceeds a reset threshold. The reset threshold may be equal to the switching threshold that is used for determining which value of the control signal is to be used. But the two threshold values may also be different.

The second value of the control signal may differ from the first value of the control signal by a value q, in particular by a value of q=2π. Due to this the amount of phase modulation fluctuates between the first value of the control signal and the second value that is shifted by the amount q. Thus, for q=2π a working point changes abruptly in dependence of the integral value by a full period of a cosine or sine shaped function, whose phase is modulated by the phase modulator. Due to the dependence from the integral value it is ensured that no control signals are fed to the phase modulator that have a low frequency spectrum, for which reason the rotation rate determination of small rotation rates in a fiber-optic gyroscope can be improved. For a value of q equal to 2□ it is further ensured that working points having the same gradient are selected.

Here, the input signal may be a superposition of a square wave signal and a ramping signal. This ensures that by jumps between different values of the square wave signal large phase shifts are generated by the phase modulator, which may e.g. be used for shifting working points. The ramping signal is superposed to the square wave signal and guarantees a non-varying phase shift for keeping a given working point. This is in particular then the case when a phase difference occurs in the interferometer. In addition, an only slowly rising ramp contributes also to the low frequency parts of the modulation signal, and hence to the lock-in effect.

The ramping signal also ensures that the integral value becomes larger over time such that the conditions for the selection of the first or second value of the control signal change. Due to this, the lock-in effect caused by the ramping signal can be reduced to a large extent, since by jumping between the first and the second value of the control signal low-frequency components are transformed to higher frequencies. Differently stated, the ramping signal obtains additional, abrupt voltage changes by switching between the first value of the control signal and the second value of the control signal, which lead to a shift of the frequency components to higher values.

The control signal may have a word length that is wider by 1 bit than the word length of the input signal, and may differ from the input signal only by an additional bit. Also, the value of the additional bit may determine whether the control signal takes on the first value or the second value. This allows a particularly simple data transfer structure and data structure.

Further, the additional bit may have a value of 0 in the control signal having the first value and a value of 1 in the control signal having the second value. The first value corresponds then to the value of the input signal and leads to the same phase modulation that would have been caused by the input signal. The value 1 of the additional bit may indicate that the integral value has exceeded the switching value and may serve to cause a phase modulation according to the value of the input signal that is reduced by q. Thus, the second value corresponds to the value of the input signal reduced by q. Due to this switching between a value of the control signal corresponding to the input signal and the value of the input signal that is reduced by the amount q, which depends on the integral value, the low-frequency part of the phase modulation signal and hence the lock-in effect is reduced or eliminated.

The phase modulator may comprise a multifunctional integrated optical chip (MIOC) that receives the control signal. In such a known MIOC the phase of passing light is modulated by using electrodes. In this process, by means of an electric field applied by using the electrodes, the effective index or the capability of guiding light can be influenced. This allows modulating the phase of passing light in a particularly simple manner.

A fiber-optic gyroscope may comprise the control system described above. In addition, the fiber-optic gyroscope may comprise a light source for emitting light having a predetermined wavelength and a beam splitter for splitting the light from the light source into two incoming beams that are guided into the phase modulator and for superposing two outgoing beams that come out from the phase modulator into a detection beam. Further, the fiber-optic gyroscope may comprise a coil for letting the incoming beams that were received from the phase modulator run into opposite directions before they are fed again as the two outgoing beams to the phase modulator, and a detector for measuring an intensity of the detection beam, which intensity is transferred to the control unit. Here, the control unit determines from the measured intensity a rotation rate around a central axis of the coil and the control signal.

This means that in the fiber-optic gyroscope a light source generates a light beam, i.e. a light signal, having a predetermined wavelength, e.g. a laser beam with a predetermined wavelength. This beam is coupled into a light guide and fed to a beam splitter that splits it into two beams that go into the phase modulator. As mentioned above, the phase modulator may be a MIOC. The beam splitter may also be integrated directly into the MIOC, e.g. such that the MIOC comprises a Y-shaped light path, e.g. two in/outputs at the one side and one out/input on the other side.

In the phase modulator the phase of the two beams with respect to each other is shifted by a predetermined value by the respective value of the control signal, before the beams are coupled into the coil. The coil is formed such that it consists of at least one light guide that is wound in a plane. In this light guide the beams are coupled such that the two beams run in opposite directions through the coil, i.e. one beam runs clockwise, while the other one runs counter-clockwise.

If the coil is rotated around a central axis that is perpendicular to the winding plane of the coil, while beams running through the coil, the effective light path of the light beams will change. The light path of beams running against the rotation direction is shortened, while the light path of beams running with the rotation direction is elongated. Light beams that have at input into the coil the same phase have at the output from a the coil a phase difference with respect to each other that is proportional to the rotation rate (Sagnac effect). This phase difference can be deduced from the interference pattern obtained by superposing the light beams. In the present case the phase difference of the incoming beams that is generated by the phase modulator is additionally changed by the phase difference resulting from the Sagnac effect.

In order to generate a readable interference pattern the outgoing beams are coupled again into the phase modulator and to the beam splitter that superposes the beams. In the phase modulator an additional phase shift is modulated to the beams depending on the respective control signal. Due to the reciprocity of the setup for a non-varying control signal the phase shifts between the oppositely running beams would vanish after one cycle period.

The phase modulation is generated in order to choose specific working points for measuring the interference signal. To allow a determination of the rotation rate, which is as good as possible, the working points are e.g. selected such that they are points of maximum gradient of the interferometer characteristic, and such that points of positive and negative gradient are selected alternately. To avoid autocorrelation in the readout circuit of the fiber-optic gyroscope the change between the working points must not be periodic, and at least four working points need to be used.

As for non-varying control signal the phase shifts of light beams going into and coming out of the coil compensate, the control signal may be very temporally such that the desired working points are obtained.

The accordingly modulated interference signal consisting of the superposed beams is coupled again via a light guide into the detector, where its intensity is measured at the working points determined by the phase modulation. From this intensity measurement the applied rotation rate and the control signals necessary for controlling the phase modulation are determined in the control unit. Phase modulator, detector, and control unit form therefore a closed control loop.

By using the setup described above it is therefore possible to provide a fiber-optic gyroscope that does due to being controlled by a control signal not have a lock-in effect for rotation rates near zero, which is caused by the frequency response of a phase modulator.

The fiber-optic gyroscope may here as described above by means of the phase modulation signal modulate the phases of the two incoming beams and the phases of the two outgoing beams such that the intensity of the detection beam can be measured at predetermined working points. This ensures that optimum measurement conditions are present, due to which the precision of the fiber-optic gyroscope is improved.

Here, the working points may be consecutive points at maximum gradient of the intensity of the detection beam plotted against its phase, and 8 working points may be used. As described already above by selecting at least four working points (readout range of the modulation signal of 2□) the risk of autocorrelation in the readout circuit of the fiber-optic gyroscope is reduced. The selection of eight working points, i.e. a readout range of the modulation signal of 4□□ leads to larger degrees of freedom for determining parameters in the control loop of the fiber-optic gyroscope. This allows satisfying all conditions on the working points for a highly precise operation of the fiber-optic gyroscope also for small rotation rates.

The fiber-optic gyroscope may be a fiber-optic Sagnac interferometer. Then, the above-described advantages are also available for measurements with a fiber-optic Sagnac interferometer.

A method for phase modulating a phase modulator of a fiber-optic gyroscope comprises the following steps: generating a control signal by the value of which the phase is modulated; determining an integral value of an integral over an input signal; supplying the first control signal to the phase modulator. Here, the control signal takes on, depending on the integral value, a first value or a second value. Due to this, a phase modulator can be controlled such that a lock-in effect is reduced.

These and further advantages of the invention are described in the following based on examples by using the accompanying figures. It shows:

FIG. 1A a schematic block diagram of a control system according an embodiment;

FIG. 1B a schematic block diagram of a control system according to a further embodiment;

FIG. 2 a schematic block diagram of data transfer in a control system according to an embodiment;

FIG. 3 a schematic block diagram of a fiber-optic gyroscope according to an embodiment;

FIG. 4 a schematic block diagram of a digital readout circuit according to the prior art;

FIG. 5 a schematic block diagram of data transfer in an control system according to an embodiment;

FIG. 6A to 6C results of rotation rate measurements using a control system according to the prior art and control systems according to the embodiment of FIG. 5;

FIG. 7 a schematic flow diagram of a method according to an embodiment.

FIG. 1A illustrates a control system 100 that is suitable for controlling a fiber-optic gyroscope. The control system 100 comprises a phase modulator 110, a control unit 120, and an integrating unit 130.

The phase modulator 110 is configured to modify the phase of a signal 115 that passes through it. For example, the phase modulator 110 allows shifting the phase of a passing light beam by a predetermined amount. The phase modulator 110 may for example comprise a multifunctional integrated optical chip (MIOC). In the MIOC the phase of light is modulated by means of electrodes. Here, by using an electric field applied by means of the electrodes the effective index or the capability for guiding light can be influenced. This allows modulating or shifting the phase of passing light in a particularly easy manner.

From the control unit 120 the phase modulator 110 obtains a control signal 125 by which the phase modulator is controlled. Besides providing the control signal 125, the control unit 120 may have also further functions. For example, the control unit 120 may be a computer processor such as a CPU that controls all or a part of the processes in a system or a device such as a fiber-optic gyroscope in which the control system 100 is used.

The amount of phase modulation is determined by the size of the control signal 125. For example, in the MIOC the control signal may be the voltage to be applied at the electrodes and the phase shift may be directly proportional to the voltage. The constant of proportionality is the electro-optic amplification factor of the used modulation structure. This factor can be determined for the used modulation structures. If one scales a control signal 125 with the inverse of this electro-optic amplification factor, the size of the resulting control signal 125 will therefore be equal to the phase shift, i.e. the value of the control signal 125 will directly indicate the phase shift. The compensation of the electro-optic amplification factor may for example be achieved by a digital analog (DA) conversion within the control unit 120.

The control signal 125 that is supplied to the phase modulator 110 may be generated by the control unit 120 based on an input signal 127. The input signal 127 is itself suited to control the phase modulator 110, but may also be processed by the control unit 120 to the control signal 125.

The input signal is not only supplied to the control unit 120, but also to the integrating unit 130. The integrating unit 130 forms the temporal integral over the input signal and determines in this manner an integral value. The integral value is transmitted from the integrating unit 130 to the control unit 120. It is a measure for the temporal growths of the input signal 127.

Based on the integral value the control unit 120 determines the value of the control signal such that the control signal 125 takes a first value or a second value. Here, the control signal 125 may be zero mean. The integral value and therefore the temporal behavior of the input signal decide thus whether the control signal 125 takes the first value or the second value. Hence, the frequency of jumps between the values of the control signal 125 is determined by the temporal behavior of the input signal 127. A result of these characteristics of the control 125 is that in the frequency spectrum of the control signal 125, which takes over time a combination of the first and the second value, frequencies near zero are strongly suppressed. Due to this, it is possible to operate phase modulators such as MIOCs with the control signal 125, whose response behavior for small frequencies differs from their response behavior for large frequencies. For example, small electro-optic amplification in the MIOC at small frequencies can be made unrecognizable by using the first or the second value in dependency from the integral value. Due to this, the signal that is used for controlling the phase modulator 110 does not have frequency components in the frequency range that is less amplified in the MIOC or comprise such components only strongly suppressed. This allows stable and reliable operation of a phase modulator having different response behaviors at small and large frequencies.

The phase modulator 110 may for example obtain the control signal 125 with the second value, when the integral value exceeds a predetermined switching threshold. Then, switching between the first and the second value of the control signal 125 depends on the behaviour of the integral over the input signal 127. For a quickly or monotonically rising input signal 127 the integral value will quickly reach the switching threshold and it will therefore quickly be switched from the first value of the control signal 125 to the second value.

The integral value may be reduced in addition by a predetermined amount by the control unit 120. This may be the case when the integral value exceeds a predetermined reset threshold. Depending on the interplay between reset threshold and switching threshold it may be adjusted how often the second value of the control signal 125 shall be used. Typically, reset threshold and switching threshold will be equal. If this is the case, the second value of the control signal 125 will be used only during a time step during which the integral value exceeds both threshold values. Due to the reduction of the integral value the first value of the control signal 125 will already be used again in the next time step. Caused by the change between the first value to the second value and back to the first value of the control signal 125 low-frequency components of the entire control signal 125 can be reduced, and therefore also the lock-in effect.

The control signal 125 may be generated from the input signal 127. For example, the first value of the control signal 125 may correspond to a value of the input signal such that the phase modulation or phase shift caused by the control signal 125 that takes the first value is equal to the one that would have been caused by the input signal 127. In addition, the phase modulation that is caused by the control value 125, when it takes on the second value, may cause an additional phase shift. The second value of the control signal 125 may for example be shifted (i.e. increased or decreased) with respect to the first value of the control signal and the phase shift due to the input signal 127 by the value q. Here, q=2π may apply, i.e. the control signal 125 having the second value causes an additional modulation by a whole period.

By shifting the phase, and thus the working point, in such a manner low-frequency parts of the input signal 127 are eliminated. Jumping between different working points acts like a change of low-frequency components of the frequency spectrum to components with higher frequencies. Thus, the frequency spectrum used for controlling the phase modulator is shifted towards higher frequencies, which reduces the lock-in effect.

The input signal 127 may for example be a superposition of a square wave signal having at least four temporarily fixed signal values and a ramping signal. Here, the temporarily fixed signal values of the square wave signal are used to generate by jumping between different signal values a sudden, discontinuous phase shift in the phase modulator 110. The temporal period during which the respective signal values are applied, and the order of the different signal values may be predetermined or may be statistically varying. In contrast, the ramping signal is used to generate continuously growing phase shifts or to compensate phase shifts acting from outside onto the signal 115. The ramping signal may in particular be used to compensate a constant phase shift that occurs e.g. due to the Sagnac effect between the oppositely circulating light beams. Because of the differentiating effect of the interferometer on the modulation signal, which effect is caused by the reciprocal light path in the coil, the ramping signal is suitable to compensate constant phase shifts.

Since the ramping signal increases relatively slowly, it is responsible for a large part of the low-frequency spectrum of the input signal 127. But at the same time, the ramping signal causes an increase of the integral value. If the integral value becomes larger than the switching threshold, an additional shift having an amount of q will be carried out. Due to this, the frequency spectrum of the input signal 127 is shifted towards higher values.

Exemplarily, FIG. 1B shows a schematic block diagram of the control system 100 in which the integrating unit 130 comprises a register 135 and an adder 137. The input signal 127 is fed in a first time step into the register 135. The integral value is equal to the value of the input signal 127 in this first time step. In the next time step the value stored in the register 135 is added by the adder 137 to the newly received input signal 127 and the obtained value is stored in the register 135. The newly stored value is indicated to the control unit. By iteration of these steps over time the integral over the input signal 127 can be formed, on which the selection of the first or the second value of the control signal 125 is based.

The control system 100 is suitable to be used in a fiber-optic gyroscope. In a fiber-optic gyroscope the phase modulation is used to carry out the measurements necessary for the determination of rotation rates and specific working points of an interferometer characteristic that indicate the interference of two light beams superposed in the fiber-optic gyroscope.

Regarding the selection of the sequence of these working points there exist some degrees of freedoms that has to be used appropriately. Selecting these working points by the phase modulator has the following goals:

-   1. Without modulation peaks of the cosine shaped interferometer     characteristic would be selected that have zero gradient. Hence, the     sensitivity of the fiber-optic gyroscope would be zero and no     directional information would be present. In order to avoid these     disadvantages, points having a maximum gradient are selected. -   2. If only points with slopes having the same sign were selected, an     applied rotation rate would lead to a DC voltage signal that would     be suppressed during consecutive amplification. Hence, working     points having alternating signs are selected. In this manner a     readout signal is created that is within the pass band of the     following amplification units. -   3. If the working points were selected such that positive and     negative gradients alternate periodically, a correlation between the     control signals and further signals that are necessary for operating     the fiber-optic gyroscope would emerge, which would lead to     insensitive bands for small rotation rates. Hence, the sequence of     the signs of gradients of the working points (modulation) has to be     selected such that this correlation becomes zero. -   4. The modulation must be carried out such that the electro-optic     amplification factor is compensated for arbitrary input rotation     rates of the sensor as was described above.

FIG. 2 illustrates a circuit-wise realization of generating the control signal 125 that takes on the first value and the second value that allow satisfying these conditions and by which the lock-in effect can be additionally reduced. The input signal 127 may be time-dependent and chosen from the range x(t)∈[0,q). the control signal may e.g. have a word length of 12 bit, wherein the weighting of the “most significant bit” (MSB) is q/2.

As described above, the input signal will be stored in a first time step in the register 135 and will be integrated over time by interaction with the adder 137. The previous register value that is added in each time step, i.e. the integral value of the previous time step, is indicated in FIG. 2 by r and has the same word length as the input signal 127.

Depending on the momentary integral value (x+r) it is determined whether the first value of the control signal 125 or the second value of the control signal 125 shall be used for phase modulation. If the momentary integral value (x+r) exceeds the switching threshold, e.g. the allowed range q, this will be signalled by the occurrence of an additional bit. It is not necessary to use the momentary integral value itself. If one complements the 12 bit of the input signal 127 having the value x with this carryover as a new MSB and interprets the 13 bit number obtained in this manner as two's complement number one obtains as control signal 125 having the first value the input signal (12 bit) without set additional bit and as control signal 125 having the second value the input signal 127 that is shifted by the amount “−q”, since the set additional bit which indicates the carryover over the switching threshold has as new MSB the weight −q. This is indicated schematically in FIG. 2 by the arrow designated with “−q”, which is intended to indicate that by the additional bit a shift of the phase modulation from x to (x−q) is achieved.

At the same time a reduction of the register entry from (x+r) to (x+r−q) is carried out. This is done automatically, since the range for x, and hence for the register 135, is restricted to the interval [0,q). Values that are larger than q will therefore considered “modulo q”, i.e. reduced by multiples of q. Due to this processing of the input signal 127 the control signal 125 will become zero-mean. The circuitry illustrated in FIG. 2 may e.g. be realized by a delta-sigma-modulator.

The input signal 127 may also have a different word length. Then, the input signal 127 is also complemented by the carryover as MSB to obtain the control signal 125, for which reason the control signal has one bit more than the input signal 127.

Depending on whether in the integration of the input signal 127 a carryover has occurred, the control signal 125 having the first value x or the control signal 125 having the second value is then supplemented to a DA converter 150 that controls in turn the phase modulator 110. Here, the value of q may be q=2□□ From the range for x∈[0,q) and the possible shift to “−q” (for x=0) a dynamic range of the phase modulator 110 of 4□ (called 4□ modulation in what follows) results.

According to a further embodiment the control system 100 described above is used in a fiber-optic gyroscope for determining rotation rates, e.g. in a Sagnac interferometer. FIG. 3 illustrates such a fiber-optic gyroscope 200.

The design of fiber-optic gyroscope 200 of FIG. 3 corresponds to the typically used design. A light source 201 emits light of the wavelength □ and frequency □□=2□c/□, wherein c is the speed of light.

The light waves travel through a coupler 202 and are then split in a beam splitter 203 into two partial beams. Both partial beams run through a phase modulator 210 that provides an additional phase modulation. Due to this between the two beams a phase shift of −φ(t)=−c₁·u₁₀₀ (t) is generated. Here, u_(φ) is a control voltage of the phase modulator 210 and c₁ its electro-optic amplification factor. The negative sign for the resulting phase difference is selected arbitrarily.

Then, both beams travel in opposite directions to each other through a fiber having total length L₀ that is wound to a coil 204 having radius A and that rotates by angular velocity □ with respect to inertial space. Due to the Sagnac effect an additional phase shift of φ_(s)=□·S with S=4□RL₀/(□c) between the two beams occurs. The runtime of light through the fiber coil be T₀. After both beams have passed through the coil 204, there is a phase shift between them of □(t)·S−φ(t−T₀). Both beams travel then again through the phase modulator 210, but this time with exchanged function such that as further component the phase φ(t) is added with positive sign. The two beams going out of the phase modulator 210 are then brought to interference in the beam splitter 203 with a total phase shift of Ω·S+φ(t)−φ(t−T₀).

After its unification the light wave travels as a detection beam again to the coupler 202 where a part of the detection beam is guided to the detector 205. There, a readout voltage u_(det)=c₀ cos(□·S+φ(t)−φ(t−T₀)) is generated that depends on the phase shift of the interfering light beams (in the formula this is indicated reduced by the DC-component of the interferometer characteristic, eliminated by a high-pass filter). The constant c₀ depends on the mean light power at the receiver, on its sensitivity, and on the amplification in following units.

The remaining part of the circuitry in FIG. 3 serves as control unit 220 and has the purpose to bring the Sagnac interferometer 200 into a state that allows an evaluation of the detector signal u_(det) for the purpose of determining the rotation rate □ by supplying appropriate signals to the phase modulator 210.

The signal u_(det) generated by the detector 205 is supplied to a first amplification unit 221 having an adjustable amplification a₀. Due to this, the signal is brought to a defined level a₀u_(det) and afterwards digitalized by an AD converter 222. The obtained signal x_(AD) is provided to a digital evaluation circuit 223 that generates a signal y_(DA). This signal, which corresponds to the input signal 127, is converted in a DA converter 224 into an analog voltage and after multiplication by an adjustable amplification factor a₁ supplied to the phase modulator 210 at a second amplification unit 226. For adjusting the amplification advantageously a multiplying DA converter is provided, whose reference voltage is used for influencing the amplification, i.e. the modulation signal is adapted to the electro-optical amplification factor c₁ by the second amplification unit 226.

Typically, the electro-optical amplification factor c₁ is compensated by the DA converter 224 and the second amplification unit 226.

The digital evaluation circuit 223 and the DA and AD converters 222, 224 operate with clock cycle T₀, which is the runtime of the light through the coil 204. Therefore, there is a closed signal path. The digital evaluation circuit 223 provides at specific, selectable times output values y_(□) for the rotation rate, y_(a0) for the amplification factor a₀ of the input path, and y_(a1) for the amplification factor a₁ of the output path. All these values are averaged values that are provided to a processor 227 for further processing. In addition, the digital evaluation circuit 223 is provided with a “clear” command after each readout of the averaged output values by the processor 227 or a timing circuit, which serves for resetting the internal averaging unit.

The processor 227 calculates from the pre-averaged values y_(□), y_(a0), and y_(a1) after optional further filtering the measurement value □ and the digital signals necessary for adjusting the amplification factors a₀ and a₁, which influence via a first supporting DA converter 228 and a second supporting DA converter 229 the corresponding first and second amplification units 222, 224. The amplification factors a₀ and a₁ may also be adapted purely digitally, e.g. by multipliers of the evaluation circuit 223, which are adapted to this end.

A digital evaluation circuit 223 according to the prior art is sketched in FIG. 4.

Since via a₁ and c₁ a relation of the digital data word y_(DA) and the optical phase φ is established, it is possible by appropriately choosing a₁ such that a₁c₁=1 holds to achieve that the single bits in the data word y_(DA) corresponding to the input signal 127 corresponds to phase shifts φ at the modulator having the size □·2^(k). In order to simplify the following explanations these values □_(k)=□·2^(k) are directly associated to the place values of the bits of the digital data word. Except for y_(DA), this definition shall also apply to all digital phase words of the digital evaluation circuit 223, i.e. also for the data words s_(i), i=1, . . . , 8, s′₃, s′₅, y_(a0), y_(a1) and y_(□) in FIG. 4. This means that in deviation from the convention the numerical value of a data word s having the bits □_(k), k=1, . . . , m is calculated according to

${s = {\sum\limits_{k = 1}^{m}{\alpha_{k}\gamma_{k}}}},{\gamma_{k} = {\pi \cdot {2^{k}.}}}$

Here, □₁ is the “least significant bit” (LSB) and □_(m) is the MSB of the data word. For the data word y_(DA) having the bits □′_(k),k=1′ . . . m′ holds

$y_{DA} = {\sum\limits_{k = 1^{\prime}}^{m^{\prime}}{\alpha_{k}^{\prime}{\pi \cdot {2^{k}.}}}}$

Since φ=a₁c₁y_(DA) for a₁c₁=1 the phase shift at the phase modulator 210 equals φ=y_(DA). Then, it holds in this case that

$\varphi = {\sum\limits_{k = 1^{\prime}}^{m^{\prime}}{\alpha_{k}^{\prime}{\pi \cdot {2^{k}.}}}}$

As will be explained, m′=0.

The input signal x_(AD) provided by the AD converter 222 is fed as internal signal s₁ to an input of the adder ADD₁. Here, depending on a demodulation signal d′₂ that can take the value 0 or 1, a weighting with 1-2d′₂ is carried out, i.e. with +1 or −1. The demodulation signal d′₂(i) is the modulation signal d₂(i) that is retarded by n cycles by retarder V₂ and generated by a random number generator M, i.e. d′₂(i)=d₂(i−n). The parameter n is adjustable within predetermined limits and serves for a runtime adaption to the external signal path. The signals d₂ or d′₂ can each assume two states (0 or 1). For d′₂=0 an addition occurs at the unit ADD₁, while for d′₂=1 a subtraction of the value s₁ occurs. The other input of the adder is connected to a register pair RP₁ into which two predetermined values +d and −d are stored. The test parameter ±d is fed as an additional signal into the main control loop as will be indicated later with the aim to “measure” its loop amplification and to control this amplification by means of a supporting control loop that influences a controllable amplifier to a predefined set value. The test signal ±d superposed to the used signal has to be sufficiently small in order to avoid overdriving of the external gyroscope path. As will be explained, for correctly adjusted amplification an exact compensation of this test signal occurs such that the measurement position of the sensor is not influenced. For selecting the respectively desired value a “select” input s is present that is controlled by a signal d′₁. The selected value active at the adder input is (2d′₁−1)·d. Then, s ₂(i)=2d′ ₁(i)−1)·d−(2d′ ₂(i)−1)·s ₁(i).

The signal d′₁ is generated analogously to d′₂ by n-times retardation by means of V₁ of the signal d₁. The signal d₁ is generated by a random number generator D that is independent of M. The sum s₂ generated in ADD₁ is supplied to the inputs of two averaging units further described below with ADD₅ and ADD₆ as well as to the input of an adder ADD₂. The output of the adder ADD₂ is supplied to a register chain REG₁ and fed back as a signal d₁ retarded by n cycles to the other input of the adder: s ₃(i)=s ₃(i−n)+s ₂(i).

In addition, s₃ also is input to the averaging unit having ADD₇ that is explained further below as well as to the adder ADD₃. At the other input of ADD₃ the aforementioned signal d₂ that is generated by the random number generator M is fed in with significance □. To the place values having smaller significance (□2, □/4, . . . ) of the same input the selectable output of a register pair RP₂ with the pre-stored values □/2+d and □/2−d is connected. The selection is carried out by the aforementioned signal d₁ that is generated in the random number generator D. Then, s ₄(i)=s ₃(i)+π/2+d _(2π)+(2d ₁−1)·d.

From the summation signal s₄ of adder ADD₃ at the digit “tr” all bits having a significance of 2□ and higher are separated. This process corresponds to a modulo 2□ operation.

The remaining bits are fed to the input of the phase integrator consisting of ADD₄ and REG₂. The summation output s₅ of ADD₄ contains also only bits having a significance of smaller than 2□. The output is retarded by REG₂ by one cycle and fed back to the other input of the adder. The carryover bit C that is generated by the addition is provided as signal d₃ to the retardation chain V₃. Then,

s₅(i) = mod 2 π[s₅(i − 1) + mod 2π[s₄(i)]] ${d_{3}(i)} = \frac{{s_{5}\left( {i - 1} \right)} + {{mod}\; 2{\pi\left\lbrack {s_{4}(i)} \right\rbrack}} - {s_{5}(i)}}{2\pi}$

Simultaneously, at the output of REG₂ the output signal y_(DA) serving as input signal 127 is output to the DA converter.

The signals s₂ or s₃ are as described above provided to three averaging units. These are accumulators that can be reset from outside that sum the signal that is to be averaged over a predetermined time period of m cycles.

The averaged rotation rate value y_(□) is generated by accumulating s₃ with ADD₇ and REG₅:

$y_{\Omega} = {\sum\limits_{i = 1}^{m}{s_{3}(i)}}$

The adjustment parameter y_(a0) is generated by an accumulation of s₂ carried out by ADD₅ and REG₅, wherein an additional weighting of s₂ by +1 or −1 is carried out that depends on d′₁:

$y_{a\; 0} = {\sum\limits_{i = 1}^{m}{{s_{2}(i)}{\left( {{2\;{d_{1}^{\prime}(i)}} - 1} \right).}}}$

Analogously y_(a1) is generated by a weighted accumulation of s₂ by ADD₆ and REG₄ that depends on d′₃. d′₃ is the signal d₃ that is retarded by n cycles by V₃ and formed out of the carryover bit C having significance 2□ in the adder ADD₄ of the phase integrator:

$y_{a\; 1} = {\sum\limits_{i = 1}^{m}{{s_{2}(i)}{\left( {{2\;{d_{3}^{\prime}(i)}} - 1} \right).}}}$

Here, it is at first assumed that the factors a₀ and a₁ are adjusted such that a₀c₀=1 and a₁c₁=1 applies. Moreover, due to the properties of the converters also n−1 dead times shall be taken into account. Then, x _(AD)(i+n)=cos(Ω·S+y _(DA)(i+1)−y _(DA)(i)).

As shown in FIG. 4 y_(DA)(i)=s′₅(i) and y_(DA)(i+1)=s₅(i) apply. In addition, s ₄(i)=s ₅(i)−s′ ₅(i)+k·2π applies.

The deviation by k·2□ occurs due to the modulo 2□ operation at “tr”. The term k·2□ can be omitted in the argument of the cosine function due to its periodicity. Then, s ₁(i+1)x _(AD)(i+n)=cos(Ω·S+s ₄(i)).

At first, it is assumed that in both register pairs RP₁ and RP₂d=0. Then: s ₄(i)=s ₃(i)+π/2+d _(2π))

and due to cos(x+π/2)=−sin(x) as well as sin(x)=−sin(x+n) and s₁=x_(AD): s ₁(i+n)=sin(Ω·S+s ₃(i))·(2d ₂(i)−1).

On the other hand:

$\quad\begin{matrix} {{s_{2}\left( {i + n} \right)} = {{- {s_{1}\left( {i + n} \right)}} \cdot \left( {{2\;{d_{2}^{\prime}\left( {i + n} \right)}} - 1} \right)}} \\ {= {{s_{1}\left( {i + n} \right)} \cdot \left( {{2\;{d_{2}(i)}} - 1} \right)}} \end{matrix}$

Then, it follows: s ₂(i+n)=−sin(Ω·S−s ₃(i)

The digital evaluation circuit 223 is a closed control loop that tries to keep the controlled deviation (Ω·S+s₃(i)) as small as possible. If this value that occurs in the argument of the sine function is small, the sine can be approximated by its argument and: s ₂(i+n)=−Ω·S−s ₃(i)

or, in its z transform form: s ₂(z)=−z ^(−n)(Ω·S+s ₃(z)).

The unit following thereafter consist of ADD₂ and REG₁, has the transfer function

${\frac{S_{3}(z)}{S_{2}(z)} = \frac{1}{1 - z^{- n}}},$

and closes the control loop. From the last two equations one obtains by eliminating the term S₂(z) the relation S ₃(z)=−z ^(−n) Ω·S.

The signal s₃ is therefore proportional to the rotation rate □. The averaging unit consisting of ADD₇ and REG₅ generates therefrom the signal y_(Ω).

The previous explanation assumes that the condition a₁c₁=1 is satisfied. A specific supporting control loop is used to control a₁ until this requirement is satisfied. Here, one uses the fact that the modulo 2□ operation carried out digitally will generate an additional error signal, if in the interferometer the phase does not jump exactly by the value 2□ corresponding to the modulo operation. The phase acting at the phase detector is φ_(d)(i+1)=Ω·S+a ₁ c ₁(s ₅(i)−s ₅(i−1)).

If the product a₁c₁ deviates from its ideal value 1, to the “ideal” detector phase the phase error will be added: φ_(e)(i+1)=(a ₁ c ₁−1)(s ₅(i)−s ₅(i−1))

After demodulation this phase error occurs as additional rotation rate signal. This error signal is therefore the scale factor deviation that is modulated by s₅(i)−s₅(i−1). But it also holds: s ₅(i)−s ₅(i−1)=mod 2π[s ₄(i)]−2πd ₃(i).

The right side of this equation can be interpreted as two's complement number having the sign bit d₃. Then, d₃ is the sign of the signal s₅(i)−s₅(i−1) that modulates the scale factor deviation (a₁c₁−1). The error modulated in such a manner occurs after n cycles at connection point s₂ and may be modulated by the sign d′₃(i) that is also retarded by n cycles in order to deduce a control parameter for a₁. This is carried out by the averaging unit consisting of ADD₆ and REG₄. The additional demodulation is carried out via the ±-control input of the adder. The average signal at the output y_(a1) is therefore a measure for the deviation of the factor a₁ from its set value (given by a₁c₁=1) and is used to adjust the factor to its set value.

For the stability of the main control loop it is necessary that the loop amplification has the correct value that is determined by a₀c₀=1. In order to always satisfy this condition a supporting control loop for adjusting a₀ is provided. For □=0s₂ the signal that is retarded by n cycles is −s₃. For □=0 and a₀c₀≠1 it holds s ₂(i+n)=−a ₀ c ₀ s ₃(i)

In order to automatically find a measure for the deviation of the factor a₀ from its ideal value in the register pair RP₂ a small test value +d and −d is stored in addition to the value □/2. Due to this, in addition to s₃ a test signal 2d₁(i)−1·d is input into the adder ADD₃ that is controlled in its sign by the random number generator D. If one is interested only in the influence of the test signal, then it applies that s ₂(i+n)=−a ₀ c ₀·(2d ₁(i)−1)·d.

If in the register pair RP₁ the same test values +d and −d are stored, the test signal (2d₁(i+n)−1)·d will be added to s₂(i+n) and it applies that s ₂(i+n)=(1−a ₀ c ₀)(2d ₁(i)−1)·d.

Then, at the connection point a component of the test signal weighted with (1−a₀c₀) is present. This component is filtered by the averaging unit ADD₅ and REG₃, whose input signal s₂ is additionally weighted with the sign of the test signal. Then, the average signal y_(a0) is a measure for the deviation of the product a₀c₀ from 1 and can be used for adjusting a₀ to its set value.

Using the above-described digital evaluation circuit 223 that is integrated in the control unit 220 it is possible to determine the rotation rate □ as well as the parameters necessary for its determination. In addition, the evaluation circuit 223 outputs that the signal y_(AD) that influences the phase modulation. From this signal y_(DA) the control unit 220 generates the control signal 125.

A modification of the digital evaluation circuit 223 according to the example of the circuit-wise realization of FIG. 2, which is useable to this end, is illustrated in FIG. 5. FIG. 5 illustrates the elements D, ADD₃, ADD₄ and REG₂ of FIG. 4 that are necessary for output of the signal y_(DA).

Instead of supplying the signal y_(DA) directly to the DA converter 224, the signal y_(DA) is modified by means of an integrating unit consisting of a register 235 and an adder 237 and the additional bit resulting therefrom as described above with respect to FIG. 2 The signal y_(DA) corresponds here therefore to the input signal 127 having the value x.

The circuitry illustrated in FIG. 5 is used for a dynamic range of the phase modulator 210 of 4□, i.e. for q=2□.

The set additional bit shifts the value of the control signal 125 from the first value to the second value, and hence the used working points by an amount of 2π. This does not change the sign of the gradient of the respective working point and leaves therefore the demodulation signal unchanged. Due to this, the demodulation signal is correlation free, although a control with suppressed low-frequency components has been achieved.

In FIGS. 6A to 6C it is based on diagrams illustrated schematically how the measurement precision of a fiber-optic gyroscope at rotation rates equal to zero improves, if one uses a control system as described above.

FIG. 6A illustrates in its upper diagram the rotation rate in °/h against time for fiber-optic gyroscope having a control system according to the prior art at rotation rate 0°/h. In this case for a system functioning without error it can be expected that only statistically distributed noise occurs, i.e. that the values of the rotation rate have a Gaussian distribution. As can be clearly seen the measured curve has accumulations at the value 0°/h, i.e. a lock-in effect occurs that biases the measurement systematically. The measurement results do not follow only a Gaussian distribution. This can be also clearly seen in the lower diagram of FIG. 6A that is a histogram of the different measurement values. It can be clearly seen that no Gaussian distributed noise is shown, as a significant accumulation at 0°/h is present.

FIGS. 6B and 6C show the same diagrams for the circuitries of FIG. 5, respectively, i.e. for a control signal 125 generated as described above having q=2□, respectively. It can be clearly seen that the measurement is only influenced by statistic noise, i.e. the histogram of the measurement value is a Gaussian curve having a center at 0°/h and the measurement values do not have an accumulation at 0°/h. Similar improvements occur for comparisons of measurements that are carried out for small rotation rates such as for example for gyro compassing in which components of Earth's rotation rate of 15°/h have to be measured as exactly as possible.

FIG. 7 shows a schematic flow diagram of a method for phase modulation. At S700 a control signal is generated, by the size of which a phase is modulated.

At S710 an integral value of an integral over the input signal is determined.

At S720 the phase modulator obtains the control signal whose value takes on a first value or a second value, depending on the integral value.

This makes it possible to reduce the lock-in effect at 0°/h by means of a control system for fiber-optic gyroscope that generates a control signal or phase modulation, and to enhance in this manner the reliability of the measurement results at rotation rates close to 0°/h. 

The invention claimed is:
 1. A control system for a fiber-optic gyroscope, comprising: a phase modulator for modulation a phase of a light signal; a control unit for generating a control signal, by the value of which the phase is modulated and that is fed to the phase modulator; an integrating unit for determining an integral value of an integral over an input signal that is suitable for modulating the phase; wherein the control signal takes on a first value that corresponds to the input signal, when the integral value is smaller than or equal to a switching threshold, and the control signal takes on a second value that is shifted by a fixed amount with respect to the input signal, when the integral value is larger than the switching threshold.
 2. The control system according to claim 1, wherein the control signal takes on the first value and the second value in such a manner that it becomes zero-mean.
 3. The control system according to claim 1, wherein the control unit is configured to reduce the integral value by a predetermined amount.
 4. The control system according to claim 3, wherein the control unit is configured to reduce the integral value by a predetermined amount, when the integral value exceeds a reset threshold.
 5. The control system according t claim 1, wherein the fixed amount has the value of q=2π.
 6. The control system according to claim 1, wherein the input signal is a superposition of a square wave signal and a ramping signal.
 7. The control system according to claim 1, wherein the control signal has a word length that is by 1 bit wider than the word length of the input signal; the control signal differs only by an additional bit from the input signal; and the value of the additional bit determines whether the control signal takes on the first value or the second value.
 8. The control system according to claim 1, wherein the phase modulator comprises a multifunctional integrated optical chip that receives the control signal.
 9. A fiber-optic gyroscope having a control system according to claim 1 and comprising: a light source for emitting light having a predetermined wavelength; a beam splitter for splitting the light from the light source into two incoming beams that are fed into the phase modulator, and for superposing two outgoing beams coming out of the phase modulator to form a detection beam; a coil for guiding therein in opposite directions the incoming beams received by the phase modulator, before they are fed into the phase modulator as the two outgoing beams; a detector for measuring an intensity of the detection beam, which intensity is provided to the control unit; wherein the control unit determines from the measured intensity a rotation rate around a central axis of the coil and the control signal.
 10. The fiber-optic gyroscope according to claim 9, wherein the phase modulator is configured to modulate the phases of the two incoming beams and the phases of the two outgoing beams such that the intensity of the detection beam can be measured at predetermined working points.
 11. The fiber-optic gyroscope according to claim 10, wherein the working points are subsequent points having maximal gradient of intensity of the detection beam plotted against the phase, and 8 working points are used.
 12. The fiber-optic gyroscope according to claim 9, wherein the fiber-optic gyroscope is a fiber-optic Sagnac interferometer.
 13. A method for phase modulating a phase modulator of a fiber optic gyroscope comprising the steps: generating a control signal by the value of which the phase is modulated; determining an integral value of an integral over an input signal that is suitable for modulating the phase; supplying the first control signal to the phase modulator; wherein the control signal takes on a first value that corresponds to the input signal, when the integral value is smaller than or equal to a switching threshold, and the control signal takes on a second value that is shifted by a fixed amount with respect to the input signal, when the integral value is larger than the switching threshold. 